This invention is particularly concerned with military aircraft as opposed to civilian or commercial aircraft and even more particularly to the class of aircraft that would fall in the fighter class. As is well known, fighter aircraft as presently perceived are designed for aerial combat. Because of the nature of its flight mission this class of aircraft typically undergoes rather violent maneuvers, calling for much manipulation of the engine power lever to change thrust of the engine so as to accelerate, decelerate, climb, dive, etc. at very severe conditions.
Many fighter engines are equipped with augmentors and during aerial combat the engine operates from maximum augmented power to 100% military power (augmentor off) to idle power. The invention disclosed herein relates to engine operation between 100% military ("mil") power and idle power, and does not affect augmented power or engine operation in the afterburning mode.
When the aircraft performs combat maneuvers the pilot will typically exercise power lever movements which result in engine speed, temperature, and airflow excursions. Under such maneuver excursions the rotor speeds of the fan and high pressure compressor rotors will vary from a maximum rotational speed at "military power" (100% thrust) to a substantially lower rotational speed called "cruise" (60% thrust) or an even lower speed called "idle" (0% thrust). While these rotor speeds and thrusts are varying during maneuver excursions, the variable geometry parts of the engine are also changing. Although the exhaust nozzle area remains constant, the fan and high compressor variable vane angles are changing with rotor speed.
The relationship between fuel flow and fan rotational speed at various exhaust nozzle areas for a gas turbine engine is shown in FIG. 1. N1C-RPM, the Y axis of the graph, is the fan speed (corrected for temperature, N1C) and main engine fuel flow is the X axis. The solid line identified by reference numeral E is the normal operating line of the engine. For each exhaust nozzle (jet) area line of the parameters shown in FIG. 1, the fan rotor speed and jet area increases in the direction denoted by the arrow. As those skilled in the art will readily appreciate, as fuel flow increases along the operating line, N1C-RPM increases along a constant exhaust nozzle area operating line, X to O.
A typical method of controlling a gas turbine engine results in an engine operating line as shown in FIG. 2 on a fan map, which shows a typical engine operating line E plotted against percent fan design pressure ratio (ordinate) and percent fan design airflow (abscissa) for given lines of corrected fan speed, N1C. For typical engine transients between mil power, 60% mil and idle, the schedule would operate the engine along the operating line E from point G to H (60% mil) to M (idle). The operating line E is spaced from the stall line J and the difference between the stall line J and the operating line E for any given corrected fan speed defines the stall margin K. On a percentage basis, stall margin K is equal to (% Fan Pressure Ratio at Stall--% Fan Pressure Ratio at Operating Condition)/(% Fan Pressure Ratio at Operating Condition). The larger the stall margin the better the engine's stability, particularly during transients. As FIG. 2 shows, a throttle transient from point G to point H results in a fan airflow excursion in excess of 10%. Unfortunately, since the engine inlet is normally designed to capture enough air to provide the correct airflow through the engine at military power, reducing fan airflow below 100% causes air at the inlet to "spill" out around the engine, and the magnitude of this spillage air increases as the fan airflow decreases. This changing spillage air affects the airflow field over and under the aircraft wings which causes aircraft buffeting. Aircraft buffeting is distracting to a pilot during combat maneuvers, and causes cracking and breakage of the brackets and rails which support the weapons on the aircraft.
What is needed is a method of operating a gas turbine engine at part power below 100% mil power which does not produce the type of inlet spillage air for thrust changes below military power.